For many years it has been known to be desirable to dispose of waste in a sanitary form. More recently, the use of selected biological materials to treat waste and to remediate contamination or spills and the like has made it possible to improve the efficiency of such disposal and remediation. A wide variety of such biological materials and techniques for their use are available.
Efforts have gone into providing improved biological materials for use in waste disposal and remediation techniques, into providing improved techniques for waste disposal and remediation, and into providing improvements in sewage treatment.
Sewage is the spent water supply of a community. Typically it is about 99.95% water and about 0.05% waste material and has a high biochemical oxygen demand (BOD). Raw sewage is generally comprised of water and suspended solids, consisting of both organic materials and inorganic materials. When the raw sewage enters a sewage treatment plant, the total suspended solids (TSS) may typically be about 300 ppm. During treatment in sewage treatment plants, the solids are concentrated and separated from the water, and the water and solids are treated and disposed of separately.
Organic materials make up most of the total suspended solids (TSS) in the raw sewage. Such organic materials include, as significant components, celluloses; proteins; lipids (fats); and starches, including amyloses and amylopectins. Other organic materials may include lignins, nucleic acids, hemicelluloses, and various other polysaccharides and polymers. Most of these organic materials comprise large molecules which results in much of the suspended solids being colloidal in nature, although some of the organic materials may also be in dissolved form or in particulate form.
The organic material also includes various microorganisms, such as bacteria, algae, and fungi, which feed on the organic materials. Some of these microorganisms exude still more large molecules, creating slime and other environmentally undesirable materials.
Inorganic materials make up a relatively small amount of the TSS, generally less than 15 to 20% of the TSS.
In a sewage treatment plant, raw sewage is usually first introduced to a tank, such as an aerated grit tank, in which very large solids, such as T-shirts, shoes, and other such undesirable materials and objects, are allowed to settle out. These very large solids are typically taken to a landfill without further significant treatment.
The remainder of the raw sewage is then treated to become acceptable to reintroduce to the environment. To that end the raw sewage goes through a second settling and clarifying process in clarifiers, in which a solids moiety (the aqueous sludge) is separated from the majority of the liquid (the supernatant). The supernatant is removed from the clarifiers for further treatment, such as chlorination, to sanitize the liquid. Before the supernatant may be reintroduced to the environment, its BOD must decline to acceptable levels, often as specified by government regulations. The sanitized supernatant is eventually reintroduced to the environment.
The solids moiety, which is typically about 3 to 4% solids in liquid, is introduced into a thickener tank for settling or to a digester, in which endogenous enzymatic and microbial activities are traditionally used to reduce the solids somewhat. Generally, the solids are held in the digester for a residence time of from at least two days to the preferred minimum of seven days, and up to 10 to 20 days, to allow the endogenous enzymes and microbes to act on the suspended solids.
Supernatant is gradually removed from the digester as digestion progresses and is recycled to the clarifiers. Solids are eventually removed from the digester and are dewatered via a centrifuge or filtering, creating a cake. The cake is removed to a landfill, is applied to farmland, or is incinerated. The cake is typically about 15% to 30% TSS, and about 85% to 70% water.
In typical prior art digestion methods of the type described, endogenous enzymes and microbes are usually slow and inefficient at reducing the TSS. Thus, significant amounts of solids must be removed from the digesters and sewage treatment plants and must be disposed of at great expense and in conformance with governmental guidelines. Some municipalities spend more than $100 per ton to dispose of such solid waste.
Treating the suspended solids and preparing the solids for final disposal is one of the most expensive components of sewage treatment. It can also be the limiting step in determining the volume of raw sewage which may be treated in a sewage treatment plant. Thus, if a plant is running near capacity, decreasing the solids produced may delay the need for a new plant or additional digesters. Therefore, if it were possible to decrease the amount of solids to be treated and disposed of, this would reduce both incremental costs and could eliminate the need for capital investment in many situations.